Maths Olympiad Prep

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Geometry Difficulty 5.4 AIME, harder Prove it Soviet Union

Problem:

Does there exist a triangle in which two sides are integer multiples of the median to that side? Does there exist a triangle in which every side is an integer multiple of the median to that side?

Solution

Solution:

Answer yes, no

The obvious approach is to make the triangle isosceles. So suppose the sides are aa, bb, bb. Then the length mm of a median to one of the sides length bb satisfies: a2+b2=2m2+b2/2a^2 + b^2 = 2m^2 + b^2/2. The simplest possibility is to take m=bm = b, so a2=3b2/2a^2 = 3b^2/2. Thus if b=2b = 2, a=6a = \sqrt{6}.

Suppose we have a triangle ABCABC, with medians ADAD, BEBE, CFCF, and BC/ADBC/AD, CA/BECA/BE, AB/CFAB/CF all integers. If AD=BC/2AD = BC/2, then A=90\angle A = 90^\circ. If AD<BC/2AD < BC/2, then A\angle A is obtuse, so at least two of the medians must be equal to the corresponding sides. So wlog we have b2+c2=5a2/2b^2 + c^2 = 5a^2/2, c2+a2=5b2/2c^2 + a^2 = 5b^2/2. Subtracting, b2a2=(5/2)(a2b2)b^2 - a^2 = (5/2)(a^2 - b^2), so a=ba = b. Hence c/a=3/2c/a = \sqrt{3/2}. So the third median has length mm where a2+a2=(3/4)a2+2m2a^2 + a^2 = (3/4)a^2 + 2m^2, so a/m=8/5a/m = \sqrt{8/5}, which is not integral. Contradiction.

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